Water reducing agents reduce the amount of water needed in cement admixtures, while maintaining good processing ability and consistency. Lignin sulfonates and naphthalene sulfonate-formaldehyde condensates have long been used as water reducing agents. These conventional water reducing agents are readily available and relatively inexpensive. However, they are used in high doses.
In contrast, newly developed polymeric water reducing agents offer high performance but are more expensive to make. U.S. Pat. No. 4,814,014, for example, teaches to graft ethylenically unsaturated monomers onto a polyether. The graft copolymer is used at a low dosage. Unfortunately, it is contaminated with a large portion of non-grafted polyether and ethylenic homopolymer. Because these non-grafted polymers do not function as water reducing agents, they reduce the effectiveness of the product.
Comb-branched copolymers of acrylic acid and polyether macromonomers have been used as high performance water reducing agents (see U.S. Pat. No. 5,834,576). The comb-branched copolymers have more uniform structures compared to the graft polymers of U.S. Pat. No. 4,814,014. Consequently, they have higher water reducing ability. An added advantage of these copolymers is the improved ability to maintain “slump.” Slump retention is the workable time after the cement admixture is mixed. Commonly used polyether macromonomers include acrylates, methacrylates, and allyl ethers of polyether.
Methods for preparing comb-branched copolymers of carboxylic monomers and polyether macromonomers are known and relatively simple. In general, free radical polymerization of a polyether macromonomer with a carboxylic monomer forms a comb-branched copolymer. A continuous process for making comb-branched polymer is described in U.S. Pat. No. 6,214,958. The comb-branched polymer made thereby performs better as water reducing agent in cement compared to that made by a batch process.
One practical issue for using comb-branched polymer in cement admixture is that comb-branched polymers entrain air. Managing the air entrainment is often difficult. Hence, new comb-branched polymers are needed. Ideally, the comb-branched polymer has reduced air entrainment.